This invention relates generally to an improved electron gun system for television receiver cathode ray tubes, and in particular to a system that provides both dynamic focus and dynamic convergence. This invention has applicability to gun systems of many types and constructions, but is believed to be most advantageously applied to systems including three-beam unitized electron guns for color television cathode ray tubes that have an extended field main focus lens. The gun system according to the invention can significantly improve the performance of cathode ray tubes, especially those having a planar, tensed foil shadow mask and associated substantially flat faceplate. The gun system according to the invention is particularly useful in improving the image resolution of flat-faced cathode ray tubes in which degradation of screen corner and edge resolution is particularly troublesome.
Desired picture tube performance characteristics of color television receive systems include high resolution, picture brightness, and color purity. Resolution is largely a function of the size and symmetry of the beam spots projected by the electron guns of the picture tube. Beam spots are desirably small, round, and uniform in size at all points on the picture screen. Achievement of these ideals is difficult because of the many factors which exert an influence on beam spot configuration. As a result of such factors, beam spots that are small and symmetrical at the center point of the picture imaging field can become enlarged and distorted at the periphery of the field, for reasons which will be described.
Key factors which influence beam spot size, uniformity and symmetry in picture tubes having three-beam electron guns include the following:
(a) electron gun design, especially the focusing system; PA0 (b) cathode ray tube screen potential; PA0 (c) magnitude of beam current; PA0 (d) the "throw" distance from the electron gun to the screen; and, PA0 (e) the convergence system. PA0 U.S. Pat. No. 3,952,224 to Evans PA0 U.S. Pat. No. 3,772,554 to Hughes PA0 U.S. Pat. No. 4,473,775 to Hosokoshi et al PA0 U.S. Pat. No. 4,513,222 to Chen
The subject invention is concerned primarily with focusing and convergence.
The ability of an electron gun to form small, symmetrical beam spots is a major factor in achieving optimum resolution. The task of designing guns with this capability has become more challenging because of the reduction in diameter of the CRT neck. This physical constraint has been largely overcome by new, more effective gun designs, such as the gun having an extended field main focus lens described and claimed in U.S. Pat. No. 3,995,194 assigned to the assignee of this invention.
Convergence of the three beams of an in-line electron gun is provided in present-day television systems primarily by the self-converging yoke. This type of yoke is a hybrid having toroidal-type vertical deflection coils and saddle-type horizontal deflection coils. The yoke contains windings which produce an astigmatic field component that has the effect of maintaining the beams in convergence as they are swept across the screen. The converging effect is shown highly schematically in FIG. 1, in which an electron gun 10 is depicted graphically as emitting three beams 12, 13 and 14 which diverge from a common plane 16 to impinge on a curved screen 18. The three beams are shown as being converged at the center point 20 of the screen 18. Due to the effect of the self-converging yoke, the three beams are also caused to be in convergence at the side of the screen 18, as indicated by point 22, even though the distance that beams must travel from the plane of deflection 16 to point 22 is greater than from the plane of deflection 16 to center point 20 of the screen.
The convergence achieved is not without cost, however, as the beam spots are subject to distortion in the peripheral areas of the screen, as will be shown with reference to FIG. 3. The distortion is acceptable in conventional tubes as the benefits and cost savings of the self-converging yoke outweigh the liabilities.
However, when the screen is flat, as indicated by screen 24 in FIG. 2, the conventional self-converging yoke is unable to maintain beam convergence, as indicated by the spread of the beam spots 28 at the sides 26 of screen 24. In addition to the spread, the spots 28 will be noted as being elongated. This elongation is due primarily to the self-converging yoke. The astigmatic field component, while self-converging the beams, undesirably introduces an astigmatic deflection defocusing of the beams when the beams are deflected away from the screen center point. This effect is indicated diagrammatically in FIG. 3 by beam spots 34. The elongation of the beam spots at the peripheries of the faceplate, and the relative increase in spot size, is indicated in greater detail in the inset figure, FIG. 3A. The beam spots 34 will be seen as comprising a bright core 34A, and transverse to the core, a dim "halo," 34B. The center beam spot 36 is shown to illustrate the magnitude of the spot size increase and distortion at the screen corner. Attempts to focus such beams are largely ineffectual due to the astigmatic effect--focusing merely results in what appears to be a "rotation" of the spot in that the core becomes the halo and the halo becomes the core.
As has been noted, the effect is tolerable in conventional tubes where the screen is curved, as shown by FIG. 1, and it is acceptably within the capability of the self-converging yoke to converge the beams without undue distortion. However, when the screen is flat, as indicated by FIG. 2, the astigmatic effect of the self-converging yoke is no longer tolerable, especially in high-resolution cathode ray tubes. Any attempt to further modify the configuration of the self-converging yoke field to adapt it to a flat screen will inevitably increase distortion outside the limits of acceptability. The self-converging ability of the yoke was already stretched to its limit in its use with the curved screen before the advent of the flat tension mask tube.
Prior art structures for statically converging electron beams have relied upon a variety of techniques such as the use of magnetic influences within and/or outside the tube envelope, and the use of electrostatically charged plates. Also, the prior art shows many examples of causing static beam convergence by inducing an asymmetry in an electrostatic field formed at the interface of two spaced electrodes. Prior art techniques for inducing electrostatic field asymmetry have included offsettting the opposing faces of two electrodes, and slanting one or more of the opposing faces so that the space lying between is in the form of a wedge--techniques described in U.S. Pat. No. 4,058,753 of common ownership herewith, and in U.S. Pat. No. 2,957,106.
Dynamic convergence means is described in U.S. Pat. No. 3,448,316. Three in-line electron beams generated by three cathodes cross over in the electrostatic field of a main lens. The center beam (green) follows a straight-line path, but the two outer red and blue beams exit the lens in divergent paths. The beams paths are reflected convergingly by electron mirrors located beyond the beam's exit from the gun. The potential on two outer plates of the mirrors is adjustable to provide for static convergence of the red and blue beams at the shadow mask. The center beam is unaffected as the potential on two inner plates through which it passes is left unchanged. Dynamic convergence is attained by changing the convergence control voltage on the outer two plates at the horizontal scanning frequency. The waveform of the convergence voltage is in the form of a parabola.
In U.S. Pat. No. 4,520,292 von Hekken et al discloses means formed in the screen grid of an electron gun for urging the outer two beams of a three-beam electron gun into convergence with the center beam. The screen grid configuration includes a transversely disposed recessed portion having a substantially rectangular center portion and substantially triangular end parts. The total effect is to the tilt the field lines within the recessed portion so that the outer beams converge.
In U.S. Pat. No. 4,058,753, of common ownership herewith, there is disclosed a three-beam electron gun for a color cathode ray tube having an extended field main focus lens means. The focus lens means has for each beam at least three electrodes including a focus electrode for receiving a variable potential for electrically adjusting the focus of the beam. In succession down-beam, there are at least two associated electrodes having potentials thereon which form in the gaps between adjacent electrodes significant main focus field components. To adjust beam focus, the strength of a first of these components is controlled by adjustment of the voltage received by the focus electrode. The strength of the second of the field components is relatively less than that of the first component. Each of the lens means is characterized by having addressing faces of the associated electrodes which define the second field component being so structured and disposed as to cause the second field component to be asymmetrical and effective to significantly divert the beam from its path in convergence of the beams without any significant distortion of the beam and substantially independently of any beam-focusing adjustments of the first field component. Electrode structures defined for producing asymmetric field components include a gap angled forwardly and outwardly, a wedge-shaped gap, and radially offset apertures.
Takenaka et al in U.S. Pat. No. 4,334,169 shows embodiments of an electron gun with a three-element main focus lens (G1, G2 and G3) and outer beam converging means at the field between the center electrode (G2) and the accelerating electrode (G3) of the main focus lens. The convergence means comprise offset apertures and apertures lying at an angle with respect to the gun axis to render the field between asymmetric. The G1 and G2 electrodes are electrically linked and receive the focusing voltage. An aperture electrode is located intermediate to G1 and G2 of the main focus lens and is electrically linked to the accelerating electrode of the prefocusing section. The object is stated to be the maintenance of the pre-established convergence of the outer beams, despite changes in the focusing voltage.
Other representative disclosures having electrode structures that influence beam convergence include:
The performance of cathode ray tubes is also a function of the ability of the gun and associated systems to establish and maintain focus at all points on the screen. Conventional curved-screen, curved-mask tubes, because of the curvature of the screen, are able to attain tolerable focusing performance on all points on the screen without the need for dynamic focusing. However, tubes having a flat faceplate exacerbate the focusing problem particularly at the screen edges due to the lack of curvature of the screen. For high-performance flat-faced tubes, dynamic focusing of electron beams is a necessity.
Techniques for dynamically varying the focus of electron beams are well-known in the art. Dynamic focusing is used to cause a beam to be in focus at the sides of the picture imaging field as well as at the center of the field. The need for dynamic focusing arises from the aforedescribed arcuate scanning of the beam with relation to the relatively planiform faceplate.
Dynamic focusing of a beam can be accomplished electronically by means of a focus-control signal modulated at the scanning frequency, with the signal being applied to a suitable beam-focusing electrode. Dynamic focusing means is disclosed by Richard in U.S. Pat. No. 3,412,281. An A.C. control signal is employed which is proportional to the distortion due to defocusing inherent in tube faces, according to Richard. The A.C. control signal is converted into a D.C. control signal which may be added to the relatively high-level constant voltage of the tube focusing circuit. Other approaches to dynamic focusing are disclosed by U.S. Pat. Nos. 2,801,363 and 3,084,276.